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Lens: Nikon M Plan 20x/0.40 210/0 LWD

Vintage: 1990's?

Lens Mount: RMS

Needed Adapters: RMS to T-mount adapter and a T-Mount to Nikon adapter, easily found on eBay.

Preferred Mounting: Normal, not reversed

Filter Thread: None

Street Price: $200-$350 in good used condition. Prices vary a lot online, shop around.

Controls: None.

Aperture: Fixed.

Basic Function: Requires a bellows to set focus and magnification

Extension Magnification Working Distance
9 cm 15.95 7 mm
13 cm 20.00 6 mm
16 cm 22.80 5 mm
19 cm 25.65 4.5 mm
Est. Focal Length: 10.4 mm

Appearance:

nikon 20x LWD top view nikon 20x LWD side view

corner sharpness graphCorner Sharpness vs. Magnification:

Corner sharpness is good for this lens at all magnification levels. Not quite as good at 16:1 magnification although the lens is made to work best at 20:1.

resolution graphSharpness and Resolution vs. Magnification:

text.

This lens shows only so-so sharpness across its usable magnification range of about 16:1 to 25:1. The resolution is quite a lot better, don't let the relatively low numbers fool you. At 25:1 this lens is resolving details at a little smaller than 0.7 micrometers, the highest resolution that I have yet measured for a lens.

The numbers may be artificially a little low (resolution-wise) as my test pattern slide is not really made to work at this magnification (I don't know of one that is at present). The slant-edge pattern starts to get a little rough when viewed this up close and personally.

The reason that this lens can resolve tiny details is related to its rather large aperture. The aperture is fixed at about f/1.2. This translates into a working numerical aperture of 0.40 at 20:1 magnification.

performance:sharpness graphPerformance: Sharpness vs. Magnification:

I have 4 lines that represent levels of performance from outstanding (top) to fair (bottom). This shows where this lens fits into the hierarchy that I have created.

The Nikon 20/0.4 LWD shows very good sharpness performance across its magnification range.

performance:resolution graphPerformance: Resolution vs. Magnification:

I have 4 lines that represent levels of performance from outstanding (top) to fair (bottom). This shows where this lens fits into the hierarchy that I have created.

The Nikon 20/0.4 LWD shows a strong very good resolution performance across its magnification range.

resolving power graphResolving Power vs. Magnification:

This graph represents the smallest details that are able to be resolved by this lens at various magnificaitons. If the number doesn't get smaller as the magnification rises, there is little benefit to going up in magnificaiton with this lens. This situation is also called empty magnification.

Very high resolution, no problems.

Chromatic Aberration:

This lens show mild color fringing in the center (about 0.80 - 1.6 pixel) and severe on the periphery (9-11 pixels). Anything below 1.0 pixel is good.

Image Contrast: Image contrast is very good across the magnification range, typical for microscope objectives.

Flare: This lens shows no significant flare during testing.

Distortion: This lens shows no significant distortion during testing.

Image Samples:

About 20:1 magnification, focus stack of 128 images, resized:

sample image

crop imagePixel level crop from the image above:

Good pixel level detail for the magnification. You just can't expect things to be sharp at this level of magnification. Each pixel on the image to the right is about 0.3 micrometers.

Conclusion:

This is a microscope objective that works well as a bellows lens. It is sometimes available on eBay for a fairly reasonable price. It is fairly easy to adapt to a bellows as it has a standard RMS mount.

This lens shows so-so sharpness, but it does show very high resolution capabilities owing to its large aperture (f/1.2). The resolution comes at the price of almost no depth of field. It required me to stack 128 images to make the sample picture above. Imaging with this lens can be very tedious and requires special equipment to focus with any precision (I use a microscope stage to focus and keep the lens and camera stationary). I tend to stick to 10x objectives and lower for most duties.

The lens has good corner sharpness across the magnification range. There is quite a lot of chromatic aberration present on the periphery of the images. This is not so obvious when the images are resized down to a sane levels.

The working distance is at the lower limits of utility for imaging with reflected light. It is difficult to get light on the subject when the front of the lens is between 4 mm and 7mm from the object. A typical 20x objective will have a working distance of about 3 mm - not enough for good lighting with reflected light. This LWD (Long Working Distance) objective doubles that. There are 20x objectives out there that provide even more working distance. These are generally called SLWD (Super LWD) and ELWD (Extra LWD).

When compared to other lenses in its magnification range, 12 mm bellows lenses, this lens out-resolves all of them. The only specification that it compares unfavorably with a 12 mm bellows lens is in the chromatic aberration numbers and the color fringing is not so obvious with real-world image sizes. They tend to have comparable or minimally better working distance than this objective.